The invention relates to an apparatus for depositing a toner on an electrostatic latent image formed on a photosensitive surface, for transferring the toner image onto a transfer sheet under application of a transfer field, and for subsequently separating the transfer sheet from the photosensitive surface and conveying it.
The present inventors have previously proposed a technique in which a toner image formed on a photosensitive member is transferred onto a transfer sheet by the application of an electric field to the transfer sheet, which is then brought close to a grounded conductor belt to cause an electrostatic induction to induce an electric charge in the belt of the opposite polarity from that of the transfer sheet so that the force of attraction acting between the transfer sheet and the photosensitive member is less in magnitude than the force of attraction acting between the transfer sheet and the conductor belt, thereby allowing the transfer sheet to be separated from the photosensitive surface and held attracted to the conductor belt for conveying purpose.
In the technique described, there has been experienced a problem that the number of lines of electric force acting between the transfer sheet and the toner transferred onto the latter decreases with time by an amount corresponding to an increase in the sheet of lines of electric force extending between the transfer sheet and the conductor belt, whereby the binding force which constrains the toner particles to the transfer member diminishes to cause a reverse transfer of the toner particles to the photosensitive surface having an increased force of attraction, giving rise to the likelihood that a failure of transfer is likely to occur in an image region having an increased amount of toner deposition.